Method of degassing of molten metal



United States Patent 3,145,096 METHIED 01F DEGASSING 9F MIDLTEN METAL Charles W. Finkl, Chicago, 111., assignor to A. Finkl dz Sons (30., Chicago, llil., a corporation of Illinois No Drawing. Filed June 5, 1961, Ser. No. 114,651 8 Illaims. till. '75--49) My invention relates to the degassing of molten metal and particularly to a novel method of degassing which utilizes vacuum. More specifically my invention is directed to a method of degassing alloy steels using vacuum as the sole gas removal agent, and which will yield final included gas contents comparable to degassing processes which employ vacuum in conjunction with purging gases. This application is a continuation-in-part of my copending application, Serial No. 77,664, filed December 2, 1958; Serial No. 54,745, filed September 8, 1960 (now abandoned); Serial No. 855,442, filed November 25, 1959 (now US. Patent No. 3,084,038, granted April 2, 1963); Serial No. 15,875, filed March 18, 1960; and Serial No. 27,826, filed May 9, 1960 (now US. Patent No. 3,071,- 458, granted January 1, 1963).

Degassing of molten metal by subjecting the upper surface of a batch of molten metal, usually in a ladle, to vacuum has been well known for many years. The prior art methods have generally not been satisfactory, however, because Stratification frequently occurs. As a result of Stratification the lower layers of molten metal are degassed to only a small extent, and if the ladle is of substantial depth no degassing occurs in many portions of the ladle remote from the surface. I have discovered that with particular steels and using a particular sized metal holding receptacle and employing a vacuum within a rather low, narrow range for a precalculated time, very excellent degassing results can be obtained on alloy steels in heats up to around 40 tons in size. It is contemplated that even larger heats can be similarly treated.

As discussed in my copending application, Serial No. 777,664, best degassing results are achieved when the surface of the molten metal is relatively free of slag. This is because the primary degassing effect is the exposure of the metal to the vacuum and a layer of slag forms a blanket which prevents good exposure of the metal to the vacuum. In addition, the weight or pressure head caused by the slag reduces the effect of the vacuum on the steel and may thereby raise the absolute pressure at the metal surface to a point at which degassing is hindered.

In addition, the dust from the slag is very troublesome. If any substantial amount of slag is used, the dust evolved will foul the ejector system and the degassing chamber. Tests have shown that the dust is slightly hygroscopic and thus absorbs moisture from the atmosphere. This absorbed moisture puts an additional vapor load on the ejectors.

Another disadvantage of a slag blanket on the metal is that the life of standard ladle and stopper rod refractories is considerably shortened. It has been established that slag, when subjected to high temperatures and when exposed to a vacuum on the order needed to degas molten metal, reacts chemically with the refractory. This chemical reaction erodes the refractory.

Another disadvantage of a slag blanket on the bath of metal is that the gas load evolved from the slag overloads.

the vacuum system at the worst possible time. Experience has shown that the steel gives off the greatest volume of gas at the beginning of the cycle. The vacuum chamber or tank must, of course, also be evacuated and the greatest volume of vapor in the tank is drawn off at the start of the vacuum cycle. Thus the gas from the slag may be sutficient to overload the vacuum system and cause a shutdown which could have disastrous results.

Finally, excessive dust in any system creates a greater 3,145,096 Patented Aug. 18, 1964 'ice explosion hazard. It is highly desirable, both from the standpoint of protecting the degassed metal from undue exposure to atmospheric conditions whereby it can reabsorb deleterious gases, and for safety reasons, that the vacuum chamber be flooded with an inert gas at the conclusion of the vacuum operation. The most readily available, and a very suitable gas is nitrogen.

For all of these reasons I have discovered. that the bath of molten metal should be subjected to vacuum in the presence of minimum slag. Ideally, no slag should be present. It is almost impossible, however, to completely remove the slag before beginning the vacuum operation. For purposes of this description and in the claims, the terms minimum slag or in the absence of slag will be used to denote a condition in which as much slag has been removed from the molten metal as is feasible within the permissible temperature drop limitation of commercial operations. Experience has shown that a thin slag coverfor instance, a layer on the order of about 1-2 inchesis desirable in many applications. Such a layer acts as an insulating cover to reduce radiant heat loss from the bath during transfer of the metal from the tapping station to the vacuum tank.

I also find it expedient to add a synthetic slag to the molten metal after degassing has been completed and while the metal is still subjected to the vacuum. The slag should be composed of a material which will not readily yield deleterious gases, such as H 0 or N on contact with the steel. Addition of a synthetic slag after degassing has been completed, or substantially completed, enables the vacuum system, which preferably is a steam ejector system, to readily handle the gas given off from the slag. In effect, the post-degassing slag acts primarily as a heat insulating blanket and a gas reabsorption barrier. That is, the slag will prevent heat loss from the upper surface of the bath between the exposure of the metal to the atmosphere upon completion of degassing and extending up to the completion of the teeming operation. In addition, since the slag itself is substantially or completely degassed at the conclusion of the degassing operation, gases from the atmosphere are effectively prevented from being reabsorbed in the molten metal between the end of the degassing operation and the end of the teeming operation. In prior art methods with which I am familiar, the effect of the degassing, at least on the middle and upper portions of the ladle, is dissipated because the metal is directly exposed to the atmosphere.

For purposes of this description and as: used in the claims, the term permissible temperature drop limitation will be used to denote the time it takes the metal to cool to a temperature below which it is considered too cold for proper teeming. The permissible temperature drop limitation will, of course, vary with. the tapping temperature and other factors, such as the design of the receptacle into which the metal is tapped.

In my copending application, Serial No. 777,664, I disclosed the fact that the degassing operation is considerably hindered by the presence of aluminum (or other active deoxidizers). Aluminum is a deoxidizer. It tends to combine with oxygen to form A1 0 If the oxygen in the melt combines with aiuminum, the carbon monoxide boil, which is an important stirring agent, is considerably reduced and consequently the degassing effect is considerably reduced. It should be understood for example, also as disclosed in my copending application, Serial No. 777,664, that degassing is promoted by the carbon monoxide boil which results from the combination of free carbon and free oxygen in the melt. As the CO bubbles upwardly it creates a stirring effect which reduces stratification in the melt.

Accordingly, in my method degassing should be carried out in the absence of aluminum or other active deoxidizers. When deoxidizers sufficiently active to deoxidize the molten metal to an appreciable extent, such as aluminum or silicon, or both, are added, they should be added to the bath after the removal of the bulk of the included deleterious gases.

Another feature which is very important to the successful operation of the instant method is the conservation of as much heat as possible during the degassing cycle. It is highly desirable to degas without adding heat to the steel during the degassing cycle. Expensive apparatus must be used if heat is to be added. Generally induction heating coils, or some form of electric heating, are employed. Induction coils are almost invariably water cooled, and should a failure occur a disastrous explosion could result.

The necessity for the addition of external heat can be overcome to a certain extent by excessive superheating of the melt. Excessing superheating, however, has certain disadvantages. As soon as the metal in the furnace is heated above normal tapping temperatures, the life of the furnace refractories begins to decrease considerably, with the result, as those skilled in the art will immediately apperciate, that additional oxygen is absorbed by the steel and there is therefore more which must eventually be removed.

One of the simplest and efficient modes of conserving heat is that disclosed in my copending application, Serial No. 855,442. In that application a radiation shield is located above the upper surface of the metal. The shield should be placed as close to the upper surface of the ladle as possible so that radiant heat given off from the heat can be most efficiently reradiated back to the heat. I prefer that the shape of the radiation shield be substantially as shown in my copending application, Serial No. 777,664, because experience has shown that such a roughly parabolic contour is about as efiicient in conserving heat as a flat disk. In addition, such a radiation shield provides room for a vigorous boiling action. It will be understood that the term reradiate as used herein is a shorthand expression for indicating a condition in which heat transfer is minimized due to little temperature difiierential above the surface of the bath. When a steel shield is used, it becomes red hot during operation but its temperature is below that of the temperature of the bath. As a result, there is heat outflow through such a metal shield, though the rate of heat transfer from the bath is obviously at a lower rate than would be the case if no radiation surface were employed. When the shield is lined with refractory, heat transfer is minimized because the temperature of the refractory approaches that of the bath.

It should be understood that during operation, the boil will be so vigorous that the molten metal will splash against the refractory. The parabolic contour provides clearance to accommodate the boil and the net eifect is to permit the ladle to be filled to a higher level than is possible when a flat disk is utilized.

The vacuum to which the molten metal should be exposed and the time of exposure to the vacuum are critical to achieving satisfactory gas levels. I have discovered that final included gas results of 1-1.5 p.p.m. hydrogen, 2030 p.p.m. nitrogen and 25-50 p.p.m. oxygen can be consistently achieved. The results are very comparable to the degassing of the same steel by processes in which both purging gas and a vacuum are employed.

On the basis of my investigations I have discovered that the steel should be subjected to a vacuum on the order of about 2 mm. Hg or below if purging is not to be employed. My investigations have also confirmed that for alloy steels the time at which the steels should be subjected to the vacuum approaching 2 mm. Hg or below should fall within the range of times given by the following formula:

Further investigations on steels of the FX and 4340 type have shown that a length of time derived from the equation 2 T in minutes: Depth of bggh 1n inches) will give adequate final included gas contents. For convenience the nominal compositions of FX and 4340 (electric furnace specification) are given below:

Depth, inches Min. Max. FX Aim Heat Size,

Time Time Time tons If time permits, the steel should be subjected to vacuum for approximately twice the calculated aim time. This isespecially advantageous in larger heats.

Listed below are actual final included gas contents and the degassing time employed for FX and 4340 steels in approximately 5000 pound and 35 ton heats:

Final Gas Contents, p.p.m. (by Vacuum Time Tap Start Heat N 0. (Grade) Fusion) (Mm) Temp, Tap to Below F. Teem., 2mm. Min. Hz N2 02 L 7 31 47 2.0 3,110 18. 0 1.3/1 27 27 1. 25 4 L6 3 30 2.0 3,120 19.6 1.4 2 50 L2 39 2.0 3,155 14.9 213 1.1 25 31 (EX) (mm 171 26 38 4.0 3,120 22.1 11 2 a 2-2 11 4 23 30 21 0 3: 14: 0 1.0 24 31 5.0 3,070 25.0

(D) Duplicate analysis.

(T) Triplicate analysis.

(S) Average analysis of degassed 5000 lbs.

(L) Average analysis of degassed 70,000 lbs.

1 Time shown is minutes below 1 mm.

2 Analysis of 8640: A steel having a nominal composition of 0 .38-43, Mn .75-1.00, Si .20-.35, Ni .40-.70, Cr .40.60, Mo .15-25.

In all of the heats in the immediately preceding table, (except of course the 35 ton FX heats 213,765 and 213,773) the depth of bath was on the order of 24 inches. It will also be noted that the tapping temperature was on the order of approximately 3l00 Fahrenheit and some even below.

Another important factor to be considered in the practice of my method is the configuration of the receptacle in which the molten metal is degassed. I have discovered that best results are achieved when the height of the receptacle and its diameter are substantially equal. One of the reasons I believe that the improved results. are

achieved is because heat loss from a container of this size is at a minimum, since such a shape has the best surface to volume ratio for the degassing process. Such a container has the smallest surface area to volume ratio of any cylindrical container. It will be understood that in the vast majority of applications of my method the molten metal will be degassed in a ladle.

It should also be understood that the degassing time as derived from the aforementioned formulas will be less than the permissible temperature drop limitation of the steel when it is expressed in terms of time. This is because some time is necessarily consumed in tapping the metal into the ladle, transferring it to the degassing station, pumping down to 2 mm. of mercury, increasing the pressure to atmospheric at the completion of the degassing cycle, transferring the ladle to the teeming station and teeming into the mold or other receiving container.

Another factor which should be considered is freeboard. It is highly desirable that a substantial freeboard be provided so that the carbon monoxide boil will not throw metal completely out of the ladle. With a container suitable for holding 2 /2 tons of molten steel 1 have discovered that approximately 2 feet of freeboard is sufiicient. With a container holding approximately 35 tons of steel, 3 feet is required.

As a final modification, I find it desirable to employ a silica type ladle brick lining in the ladle. Such a lining has the following approximate analysis:

Silica 65.55 Alumina and titania 27.35 Ferric oxide 3.45 Calcium oxide .55 Magnesia .47 Alkalies and others 2.43 Ignition loss .20

I am not entirely positive of the chemical actions that take place, but I believe that the silica reacts with carbon in the melt to form CO which bubbles to the surface, thus creating a carbon monoxide boil. There will be some expansion in volume of the CO as it passes upwardly through the melt. This phenomenon is indicated during an actual degassing operation by a ring of bubbling metal around the periphery of the metal and adjacent the ladle walls. As mentioned above, this CO boil considerably reduces the tendency for stratification of the metal.

Although a broad disclosure relating to a good many alloy steels and a preferred disclosure relating to two particular types of low alloy steels have been disclosed, it will be understood that the invention is described in illustrative terms only. Accordingly, the scope of the inven tion should only be limited by the scope of the following appended claims.

I claim:

1. A method of vacuum degassing a bath of molten steel in a container within the permissible temperature drop limitation of the metal, said method including the steps of removing a substantial quantity of the slag from the surface of the bath and thereafter, in the absence of a deoxidizer sufiiciently active to deoxidize the molten metal to an appreciable extent prior to degassing, subjecting the molten metal to a vacuum approaching 2 mm. Hg or less for a period of time determined by the formula din inches 2 w to in which T=time in minutes at which the heat is subjected to the vacuum and d=depth of the molten metal in inches, and, during the subjection of the molten metal 6 to vacuum, reducing heat transfer from the bath to a minimum by maintaining little temperature differential between the upper surface of the bath and :a radiant heat intercepting surface located thereabove.

2. The method of claim 1 further characterized by and including the step of adding a layer of inert slag as a heat insulating blanket to the bath upon completion of subjection to the low vacuum.

3. The method of claim 1 further characterized by and including the step of exposing the bath to an inert gas atmosphere between the end of the vacuum treatment and the commencement of exposure of the bath to atmospheric conditions.

4. The method of claim 1 further characterized by and including the step of adding a desired addition of a deoxidizer selected from the group consisting of aluminum and silicon to the bath after removal of the bulk of the included deleterious gases.

5. A method of vacuum degassing a heat of low alloy steel having as its principal constituents a composition substantially as follows:

Mn 1.00 Si .20.35 Ni 40-200 in a ladle within the permissible temperature drop limitation of the metal, said method including the steps of removing as much slag as possible from the surface of the melt and thereafter, in the absence of a. quantity of a de oxidizer sufficiently active to deoxidize the steel to an appreciable extent prior to degassing, subjecting the steel to a low vacuum approaching 2 mm. Hg or less for a period of time determined by the formula cl in inches 2 T 20 in which T=time in minutes at which the heat is subjected to the vacuum and d depth of the steel in inches, and during the subjection of the steel to vacuum, reducing heat transfer from the heat to a minimum by maintaining little temperature differential between the upper surface of the heat and a radiant heat intercepting surface located thereabove.

6. The method of vacuum degassing a heat of low alloy steel of claim 5 further characterized by and including the step of tapping the heat into the ladle at a tapping temperature on the order of approximately 3100 degrees Fahrenheit.

7. The method of claim 5 further characterized by and including the step of adding a layer of inert slag to the heat upon completion of removal of the bulk of the included deleterious gases and thereafter exposing the heat to vacuum for a length of time sufficient to degas the inert slag.

8. The method of claim 5 further characterized by and including the step of exposing the heat to an inert gas atmosphere between the end of the exposure of the heat to the low vacuum and the commencement of exposure of the heat to atmospheric conditions.

References Cited in the file of this patent UNITED STATES PATENTS 1,131,488 Dolensky Mar. 9, 1915 1,277,523 Yensen Sept. 3, 1918 2,741,555 Cuscoleca et al Apr. 10, 1946 2,994,602 Matsuda Aug. 1, 1961 

1. A METHOD OF VACUUM DEGASSING A BATH OF MOLTEN STEEL IN A CONTAINER WITHIN THE PERMISSIBLE TEMPERATURE DROP LOCATION OF THE METAL, SAID METHOD INCLUDING THE STEPS OF REMOVING A SUBSTANTIAL QUANTITY OF THE SLAG FROM THE SURFACE OF THE BATH AND THEREAFTER, IN THE ABSENCE OF A DEOXIDIZER SUFFICIENTLY ACTIVE TO DEOXIDIZE THE MOLTEN METAL TO AN APPRECIABLE EXTENT PRIOR TO DEGASSING, SUBJECTING THE MOLTEN METAL TO A VACUUM APPROACHING 2 MM. HG OR LESS FOR A PERIOD OF TIME DETERMINED BY THE FORMULA T=((D IN INCHES)/10 TO 30)(**2) IN WHICH T= TIME IN MINUTES AT WHICH THE HEAT IS SUBJECTED TO THE VACUUM AND D=DEPTH OF THE MOLTEN METAL IN INCHES, AND DURING THE SUBJECTION OF THE MOLTEN METAL TO VACUUM, REDUCING HEAT TRANSFER FROM THE BATH TO A MINIMUM BY MAINTAINING LITTLE TEMPERATURE DIFFERENTIAL BETWEEN THE UPPER SURFACE OF THE BATH AND A RADIANT HEAT INTERCEPTING SURFACE LOCATED THEREABOVE. 